1. Field of Invention
The present invention relates to an electronic timepiece and to a time adjustment method for an electronic timepiece that receives radio signals transmitted from positioning satellites such as GPS satellites and acquires the current date and time.
2. Description of Related Art
The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on known orbits, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time or satellite time information) with extremely high precision.
All GPS satellites transmit the same GPS time, and the Universal Coordinated Time (UTC) is acquired by adding the UTC offset to the GPS time. For an electronic timepiece to receive the satellite signal transmitted from a GPS satellite, acquire the GPS time, and display the local time (regional time) at the location where the electronic timepiece is being used, the time difference to the UTC must be added after correcting for the UTC offset in order to get the current local time, and the electronic timepiece must therefore know what this time difference is.
The UTC offset can be acquired from the data in the received satellite signal, or a predetermined value stored in ROM may be used.
Radio-controlled timepieces and navigation systems that use satellite signals transmitted from GPS satellites to acquire positioning information and time information (UTC), determine the time zone (time difference) of the current location from the acquired positioning information, and calculate and display the current local time are known from the literature. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-2003-139875 and Japanese Unexamined Patent Appl. Pub. JP-A-2003-4457.
Some problems with the foregoing related art are described below.
The technology taught in Japanese Unexamined Patent Appl. Pub. JP-A-2003-139875 extracts the fixed position information closest to the mobile device position by selecting a circular area centered on a particular fixed position and uses the time difference (time zone) for that fixed position if the position of the mobile device is in that area.
In order to adjust the size of these circular areas, distances are normalized using a weighting coefficient referred to as the fixed range information. However, when the time zone borders overlap and there are multiple fixed positions around and near the location of the mobile device, it is difficult to set the fixed ranges so that detection errors do not occur, and the amount of data that must be stored increases.
Furthermore, because the distance between the mobile device and each fixed position must be calculated, computation time increases when there are multiple fixed positions in the vicinity of the mobile device, the time difference (time zone) cannot be set quickly, and user convenience is thus impaired.
The technology taught in Japanese Unexamined Patent Appl. Pub. JP-A-2003-4457 relies on using map data stored on a CD-ROM, DVD, Or other medium, acquires the time zone information based on a massive amount of map data, and therefore cannot be used in devices, such as wristwatches, with limited memory capacity.